Effect of Ignition Timing Advance on Performance of a ...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 9, Number 13 (2014) pp. 2341-2348 © Research India Publications http://www.ripublication.com

Effect of Ignition Timing Advance on Performance of a Small Producer Gas Engine N. Homdoung1, N. Tippayawong1*and N. Dussadee2 1

Department of Mechanical Engineering, Chiang Mai University, Chiang Mai 50200, Thailand. * Corresponding author E-mail: n. tippayawong@yahoo. com 2 School of Renewable Energy, Maejo University, Chiang Mai 50290, Thailand.

Abstract In this work, a small, single cylinder, naturally aspirated, compression ignition engine was modified into a spark ignited (SI) engine where producer gas was used solely as fuel. Experiments were carried out at various engine speeds and loads to study effect of ignition timing adjusted to maximum brake torque (MBT) on overall engine performance. From the tests, it was found that coefficient of variation from representative measurements was in a range of 1. 75-3. 0%. As expected, the performance of the engine was dependent on ignition timing advance. The optimum ignition timing of the small producer gas engine was observed to be between 20° to 25° BTDC at 1100 rpm, and increase with engine speed. Maximum brake mean effective pressure and minimum brake specific fuel consumption rate were 195. 48 kPa, and 0. 93 kg/kWh, respectively, obtained at 1700 rpm on full load. At this condition, brake thermal efficiency of about 19% was achieved. Keywords : Biomass, Ignition timing, Small engine, Producer gas, Renewable energy

1. Introduction Limitation of conventional fossil fuel reserves and reduction of environmental impact have intensified the search for alternative fuels in internal combustion engines. Renewable fuel is an obvious solution to this problem. Biomass derived, producer gas is an interesting source that can be the fuel of choice in the future. The producer gas derived from biomass via gasification has average composition consisting of 4-10 % H2, 28-32 % CO, 0-2 % CH4, 1-3 % CO2 and 55-65 % N2 with mean calorific value of about 4500 – 5600 kJ/Nm3 [1]. The stoichiometric air to fuel ratio is 1. 25 ± 0. 05 on

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mass basis. The laminar flame speed is in a range of 10-12 cm/s [2]. However, when use in an engine, the power output and efficiency were reported to decrease, compared to a typical liquid fuel [3]. Adjusting ignition timing may improve the engine performance. With respect to previous works on ignition timing effect on performance of SI engines, Lawankar et al. [4] tested a medium sized, SI engine with gasoline and LPG. They found optimum ignition timing of the engine to be 20° BTDC for gasoline and 30° BTDC for LPG, respectively. Gopal et al. [5] reported appropriate ignition timing for CNG and gasoline engines in which the maximum brake thermal efficiency occurred at 27° BTDC for CNG, and at 32° BTDC for gasoline. For CNG, duration of the burn was needed to increase due to slower flame speed [6]. Kakaee et al. [7] reported similar ranges to Lawankar et al [4] and Gopal et al. [5]. Shidhar et al. [2] worked on varying ignition timing of a range of SI engines with producer gas operation at high compression ratio (CR) mode. Appropriate ignition timings were identified. Works on SI engines on different gases such as methane and landfill gas [8], biogas [9] and hydrogen [10] were also available. To the authors' knowledge, it is clear that currently there is no report on small engines with producer gas operation. It is therefore the focus of this work to investigate if improvement can be achieved with adjustment of the ignition timing advance for a small producer gas engine.

2. Material & Methods 2. 1 Apparatus In this study, producer gas was generated from a downdraft gasifier, shown in Figure 1. The reactor can generate producer gas up to 27 Nm3/h. Charcoal consumption rate was between 5-6 kg/h. The gas cleaning and cooling unit consists of a cyclone, a water scrubber, an organic filter and a fabric filter. Tar and particulate matter before entering to the engine were less than 50 mg/Nm3. The modified engine was of a single cylinder type, naturally aspirated, four-stroke and water cooling, and usually employed as an agricultural powertrain. Modification of the engine was conducted on the ignition system, cylinder head, and air-producer gas mixer. The optimum CR was achieved at 14:1. Ignition system was installed in place of a fuel injection system. The ignition timing can be varied in a range of 0° to 60° BTDC. The gas mixer design was based on air-gas carburetor and operating between 1000-2000 rpm.

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Figure 1: Schematic diagram of gasifier system

2. 2 Data analysis A Shimadzu GC-8A gas chromatography machine was used to measure mole fractions of CO, H2, CH4, CO2 and N2 in the producer gas. Average chemical compositions were found to be CO = 30. 5±2%, H2 = 8. 5 ± 2%, CH4 = 0. 35%, CO2 = 4. 8±1%, and O2 = 6. 3±0:5%. Calculated calorific value of the producer gas was 4. 64 MJ/Nm3. The density of charcoal was about 250–300 kg/m3 with average moisture content of 7%. The experiment conditions were at ambient pressure of 0. 92 kPa. Average air density was 1. 1 kg/m3. Ambient temperature during the testing period was 30 ± 3°C. 2. 3 Test procedures Engine tests were carried out at varying ignition timings between 20°-50° BTDC. The engine speeds were in a range of 1100–1900 rpm on part load and full load mode. All experimental were done at the corresponding MBT. Air and fuel were tuned to achieve the maximum power. The measurements were recorded at an average interval of 10 min, after achieving a stable operation. Charcoal consumption at each load was monitored by weighing the mass of charcoal feeding into the gasifier. The producer gas and airflow rates were measured using Lutron YK-80 flow meters. F609 Chauvin Arnoux watt meter was used. Electrical load consists of ten 100W bulbs with ten 500W heaters. Temperatures of exhaust gas, water and oil lubricant were measured using type K of thermocouples connected to Yokokawa DX 220-1-2 data logger. The coefficient of variation (COV), specific fuel consumption (BSFC), brake mean effective pressure (BMEP), brake thermal efficiency (BTE), optimum ignition timing were evaluated.

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3. Results and Discussion General observation revealed that the exhaust gas temperature of small producer gas engine was in a range of 298-420°C, while water and oil temperatures were between 93-104°C. The exhaust gas, water and oil temperatures increased with increasing engine speed, due to increased fuel input to engine cylinders and sub segment increase of turbulence intensity, heat release rate, and maximum flame temperature [8]. They were stable throughout the tests. 3. 1 Coefficient of variation A COV is a measure of cyclic variability that occurs during early stage of combustion and around peak pressure. Figure 2 shows variation of COV of BMEP with engine speed at 60% load and full load. For each speed, the ignition timing was adjusted to MBT timing. The COV of BMEP was found to vary between 1. 75 to 3. 0%. Minimum COV occurred at 1300 rpm. At higher engine speeds, the COV of BMEP was found to increase, but remained small. Increase in COV was due to difference in cycle-to-cycle combustion process caused by variations in mixture motion in cylinder, the mixing of air-producer gas and residual gas in cylinder for each cycle [11]. In comparison between operation loads, the use of full load appeared to show higher COV than part load. 3. 2 Brake mean effective pressure Figure 3 shows effect of ignition timing, engine speed and load on BMEP of the small producer gas engine. The results show that BMEP tended to increase with appropriate advance ignition timing that mostly depend on engine speed and load. Except at 1500 rpm on full load, the small engine exhibited deceleration when adjusted to lower than 35° BTDC ignition timing. Retarding ignition timing, the air-fuel in cylinder will burn as the piston is moving down, leading to decreasing pressure and performance. With advanced ignition timing, the mixer in cylinder will burn while the piston is moving up in compression stroke. The best ignition timing found in this experiment on full load was 25° BTDC at 1100 rpm, 30° BTDC at 1300 rpm, 35° BTDC at 1500 rpm, 40° BTDC at 1700 rpm. At 1900 rpm, the engine appeared to show knocking. For 60% load, the best ignition timings were similar to the full load. The maximum BMEP (195. 48 kPa) occurred in full load at 1700 rpm, whereas the minimum of BMEP (64. 45 kPa) was obtained at 1100 rpm. 3. 3 Brake specific fuel consumption Figure 4 shows variation of BSFC with adjusted ignition timing, engine speed and load of the small producer gas engine. The BSFC rate tends to decrease with ignition timing. In comparison of difference load and engine speed, the minimum BSFC rate occurred on full load operation and at 1700 rpm of engine speed. Increasing engine speed tended to decrease BSFC rate. The lowest BSFC rate of 0. 93 kg/kWh in small engine was achieved. Generally, the BSFC rate of producer gas engine was in a range between 1. 2-2 kg/kWh [12].

Effect of Ignition Timing Advance on Performance 4.0

60% of load full load

3.5 COV of BMEP (%)

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Figure 2: COV of BMEP with engine speed for two different loads 250 1100 rpm (full load) 1300 rpm (full load) 1500 rpm (full load) 1700 rpm (full load) 1100 rpm (60% load) 1300 rpm (60% load) 1500 rpm (60% load) 1700 rpm (60% load) 1900 rpm (60% load)

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Figure 3: Relation of ignition timing, engine speed and load on brake mean effective pressure

Figure 4: Relation of ignition timing, engine speed and load on brake specific fuel consumption

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3. 4 Brake thermal efficiency Using producer gas in small engines adjusted to suitable ignition timing, high BTE can be obtained. Adjusting ignition timing related to combustion process in cylinder directly affected the power output and fuel consumption. Figure 5 shows BTE as a function of ignition timing, engine speed and load. Maximum BTE of 18. 8% was obtained at highest engine speed on full load. This was in similar magnitude to those from medium and large engines. Typical thermal efficiency of large producer gas engines was in a range of 18-24 % [12, 13, 14]. 30 25

1100 rpm (full load) 1300 rpm (full load) 1500 rpm (full load) 1700 rpm (full load) 1100 rpm (60% load) 1300 rpm (60% load) 1500 rpm (60% load) 1700 rpm (60% load) 1900 rpm (60% load)

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Figure 5: Relation of ignition timing, engine speed and load on brake thermal efficiency

3. 5 Optimum ignition timing Figure 6 summarizes optimum ignition timing of the small producer gas engine obtained at each engine speed on part load and full load. The ignition timing tended to increase with engine speed because the air-producer gas mixture in cylinder was turbulent due to fast moving of gas. The burning time became shorter at higher engine speeds. So, it was necessary to increase the burn duration. At 1100 rpm, maximum power output occurred during 20° to 25° BTDC. Engine speed of 1500 rpm is interesting because most applications will use this speed. The best power output was between 32. 5° to 37. 5° BTDC for 1500 rpm. It should be noted that when adjusted to 40° BTDC of ignition timing advance, the power output was reduced. At 1900 rpm maximum speed, the small producer gas engine was unable to operate at full load due to deceleration and knocking when adjusted to 40° ignition timing advance. Meanwhile, the good acceleration stability was observed at 60 % of load or lower. Therefore, the best power output on mid load was expected to occur during 40° to 45° BTDC of ignition timing advance.

Optimum ignition timing (degree)

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Figure 6: The optimum ignition timing of small producer gas engine with varying engine speed

4. Conclusions From the investigation, it was found that a small agricultural engine can operate satisfactorily well with producer gas. Adjusting ignition timing can improve performance of the producer gas engine. In this work, the optimum ignition timing of the small producer gas engine were between 20° to 25° BTDC at 1100 rpm, 25° to 30° BTDC at 1300 rpm, 32. 5° to 37. 5° BTDC at 1500 rpm and 40° BTDC of 1700 rpm. Appropriate ignition timing advance enabled BMEP to increase. The maximum BMEP of 195 kPa was achieved at 1700 rpm of full load. At this speed, the lowest BSFC rate of 0. 93 kg/kWh and maximum BTE of the small producer gas engine was achieved.

Acknowledgment The authors would like to thank the Graduate School and the Department of Mechanical Engineering of Chiang Mai University, as well as the Energy Research Centre of Maejo University for providing test facilities and technical supports. Financial supports from the Energy Policy and Planning Office, and the Commission on Higher Education were highly appreciated.

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